Evidence produced in the mid 1990's suggests that bovine spongiform encephalopathy (commonly known as mad cow disease) was probably transmitted to humans. The agents widely believed to be responsible for these diseases are abnormally shapened prion proteins. Along with an infectious route, humans can also develop prion diseases either sporadically or by expressing a mutant prion protein (penetrance approximately 100%). Recent work points to the cytosol as being the site of toxicity of prion proteins. Mutant prion proteins may be highly toxic because they have an increased probability of misfolding and therefore mislocalizing to the cytosol. To study the mechanism of increased toxicity of the mutant prion proteins and how it relates to cytosolic prion toxicity, ES cells will be genetically altered to express mutant prion proteins directly into the cytosol. These cell lines will be differentiated into neurons, gila, or fibroblasts and used to study the effects these mutations have on subcellular traffciking, degradation, and toxicity of PrP. The gene-targeting approach will allow for a high probability of equivalent expression of these constructs at the mRNA level. The selection of ES cells as a model system will permit efficient transition of cell culture experiments into mice.